The present invention relates generally to a manner by which to compensate for signal distortion introduced upon a send signal during amplification by an amplifier which forms a portion of a sending station. More particularly, the present relates to apparatus, and an associated method, by which to predistort, in a dynamic manner, the send signal to compensate for the distortion introduced upon the send signal. Compensation is effectuated by altering the phase of the send signal by a selected amount without altering the magnitude of the send signal. Operation of an embodiment of the present invention is particularly beneficial when output power magnitude modification is not generally permitted, such as in a CDMA (code-division, multiple-access) communication system.
A communication system permits the communication of information between a sending station and a receiving station by way of a communication channel. The sending station is operable to generate a communication signal of characteristics permitting its communication upon the communication channel. And, the receiving station is operable to recover the informational content of the communication signal.
A radio communication system is a communication system in which the communication channel upon which the communication signal is communicated is formed of a radio channel. The radio channel is defined upon a portion of the electromagnetic spectrum. Because a wireline connection is not required to form the communication channel between the sending and receiving stations, communications are possible when such a wireline connection between the sending and receiving stations would be impractical. Improved communication mobility is also possible through use of a radio communication system.
A sending station forming a portion of a radio communication system includes a transmitter for modulating information upon a carrier wave of a carrier frequency within the range of frequencies which defines, at least in part, the communication channel. Through such a process, a baseband signal of which the information is formed is converted into a radio frequency signal of desired frequency characteristics.
The transmitter typically includes one or more up-mixing stages at which the baseband information is up-converted in frequency to be of the selected radio frequency. The mixing stages include a mixer circuit coupled to receive the information and an up-mixing signal with which the information is to be multiplied, or otherwise combined, to form an up-converted signal. When multiple mixing stages are utilized, an if (intermediate frequency) signal is formed at a first, or first series of, mixer stages. A radio frequency signal is formed at the final mixing stage. The radio frequency signal is amplified by a power amplifier to increase the energy level of the signal. Once amplified, the signal is applied to an antenna transducer which converts the radio frequency signal into electromagnetic form for communication upon the communication channel.
A cellular communication system is exemplary of a radio communication system. Cellular communication systems, constructed according to various cellular communication standards, have been installed throughout significant portions of the world. A subscriber to a cellular communication system is able to communicate therein by way of a mobile terminal when the mobile terminal is positioned within an area encompassed by the communication system. Telephonic communication of both voice and non-voice information is permitted by way of such communication systems. The mobile terminal is formed of transceiver circuit and includes both a transmitter portion and a receiver portion.
The transmitter portion of the mobile terminal, as noted above, typically includes a power amplifier at which a send signal is amplified. Power amplifiers, however, are typically of a limited dynamic range. And, within the range of frequencies over which the amplifier is operable, the gain and phase properties of the power amplifier are typically not constant. Such variations result in signal distortion of a signal amplified by such a power amplifier and reduce the communication quality of the resultant communications.
To attempt to compensate for such distortion, various manners of signal precompensation of a signal, prior to its application to the power amplifier, have been developed. Such existing manners generally alter the magnitude levels of the signal prior to its application to the power amplifier. But, such manners by which to precompensate the signal prior to its application to the power amplifier are unavailable, typically, in a CDMA communication system. In a CDMA communication system, maintaining the output power independent of the amplifier characteristic is important for the reason that a fast feedback, power-control mechanism is the manner by which near-far interference from individual users is combatted.
Also, some existing manners by which to perform precompensation utilize a look-up table. Generally, however, use of look-up tables are cumbersome and inadequate when dynamic changes are required to be made to effectively compensate for signal distortion caused by amplification of the signal at the power amplifier.
If a manner could be provided by which better to compensate for signal distortion caused by amplification of the signal at a power amplifier, improved communication quality would result.
It is in light of this background information related to generation of a send signal at a sending station that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus, and an associated method, by which to compensate for signal distortion introduced upon a send signal during amplification of the signal by an amplifier which forms a portion of a sending station.
During operation of an embodiment of the present invention, the send signal is predistorted to compensate for the distortion introduced upon the send signal by the amplifier. Predistortion is effectuated by altering the phase of the send signal by a selected amount without altering the magnitude of the send signal. Thereby, distortion of a resultant, amplified signal is reduced without altering the magnitude of the amplified signal.
In one implementation, apparatus is provided for transmitter circuitry of a radio transmitter operable in a radio communication system. A distortion estimator is connected in a feedback relationship with a power amplifier which forms a portion of the radio transmitter circuitry. The distortion estimator is also coupled to receive indications of the send signal prior to its application to the power amplifier. The distortion estimator is operable to compare the send signal, prior to its application to the power amplifier, together with the corresponding send signal, subsequent to amplification by the power amplifier. A distortion estimate is determined therefrom. Parameters which characterize an AM (amplitude modulation)-to-PM (phase modulation) response of the power amplifier are also determined. The parameters include, for instance, the instantaneous power of the send signal. Values determined by the distortion estimator are provided to a phase rotator. The phase rotator is positioned in-line with the power amplifier and is coupled to receive the send signal, prior to its application to the power amplifier. The phase rotator is operable to alter the phase of the send signal responsive to values provided by the distortion estimator.
In one implementation, the radio transmitter circuitry forms a portion of a radio transceiver operable in a cellular communication system which utilizes a QPSK (Quadrature Phase Shift Keying) modulation scheme. In a QPSK modulation scheme, symbols of a QPSK constellation set define the informational content of the send signal generated by the radio transceiver circuitry. Each symbol includes a phase component and a magnitude component. Successive ones of the symbols of which the send signal is formed are applied to the phase rotator, and the phase components of the successive ones of the symbols are rotated to compensate for the distortion of the values of the symbols when amplified at the power amplifier. Because the phase rotator is operable responsive to values provided thereto by the distortion estimator, connected in a feedback relationship with the power amplifier, the predistortion of the signals is effectuated dynamically responsive to determinations of the levels of distortion introduced upon the symbols of the send signal during amplification thereof by the power amplifier.
In one implementation, the radio transceiver is operable in a CDMA (code-division, multiple-access) cellular communication system. Because compensation of the distortion introduced upon the symbols of the send signal is effectuated by altering the phase components of the QPSK symbols, the distortion is compensated for without altering the magnitude components of the symbols. Thereby, system requirements, generally set forth in CDMA cellular communication systems to maintain output power independent of the power amplifier characteristic is assured.
In one implementation, the phase rotator is operable to select a level of phase rotation responsive to parameters calculated at the distortion estimator pertaining to the parameters of an equation which characterizes the AM-to-PM response of the amplifier. The response is constant below a threshold input power level. And, beyond the threshold input power level, the response is linear in a log-domain. Thereby, the phase distortion for which compensation is made is constant when the input power level is beneath the threshold level. And, the phase distortion for which compensation is made is linear when the input power level is beyond the threshold. Compensation of the phase distortion is made by de-rotating the symbol by an amount to compensate for such phase distortion.
In these and other aspects, therefore, apparatus, and an associated method, is provided for a sending station operable to transmit a send signal, once amplified by an amplifier. Compensation is made for distortion introduced upon the send signal when the send signal is amplified by the amplifier. A phase rotator is coupled to receive indications of the send signal prior to amplification by the amplifier. The phase rotator selectably rotates a phase component of the send signal. A distortion estimator is coupled to receive the indications of the send signal prior to amplification by the amplifier and to receive indications of the send signal subsequent to amplification by the amplifier. The distortion estimator estimates an indicia of distortion of the send signal due to amplification thereof by the amplifier. The distortion estimator also provides a distortion estimate signal to the phase rotator, values of the distortion estimate signal determinative of rotation by the phase rotator of the phase component of the send signal.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings, which are briefly summarized below, the following detailed description of the presently-preferred embodiments of the present invention, and the appended claims.